Variations in the age, endocrine status and reproductive history of animals are known to greatly affect the incidence, latency period, and total number of mammary cancers induced by chemical carcinogens. A new mammary cancer model using the carcinogen l-methyl-l-nitrosourea (MNU) was recently developed but has not been adequately investigated with respect to the above parameters. Due to the current interest in the MNU model as a system for studying the effects of nutrition and chemopreventive agents on mammary carcinogenesis and because of the association of nitroso- compounds and human cancer, it is important that studies be performed which further characterize this model. Mammary cancers will be induced in female Sprague-Dawley rats by administering two intravenous injections of MNU seven days apart. At 180 days after the initial MNU injection, the animals will be sacrificed and the incidence of mammary cancer, the average number of cancers per rat, the latency period of the cancers, and the incidence of cancer invasion and/or metastasis will be determined. Specific studies are proposed to evaluate the effects of ovarian steroids, pregnancy and lactation, age at time of carcinogen treatment, previous reproductive history, and age at first pregnancy on MNU-induced mammary tumors in dams and in offspring when MNU is administered during pregnancy. The data obtained will further confirm that the MNU model is superior to other experimental mammary cancer maodels for future studies of human breast cancer. Furthermore, the investigation will determine the response physiologically altered mammary tissue to a different type of carcinogens (i.e., nitrosamides) and demonstrate any relationship between hormones and the potential of these cancers to invade or metastasize.